We regularly encounter the situation that devices are subject to
changes that will make them incompatible to previous versions.
Removing SUPPORTED_DEVICES will not really be helpful in most of these
cases, as this only helps after a rename.

To solve this situation, this patchset introduces a compatibility
version for devices. In this patch, the actual checks are implemented
into fwtool_check_image():

If an incompatible change is introduced, one can increase either
the minor version (1.0->1.1) or the major version (1.0->2.0).

Minor version increment:
This will still allow sysupgrade, but require to reset config
(-n or SAVE_CONFIG=0). If sysupgrade is called without -n, a
corresponding message will be printed. If sysupgrade is called
with -n, it will just pass, with supported devices being checked
as usual. (Which will allow us to add back SUPPORTED_DEVICES for
many cases.)

Major version increment:
This is meant for potential (rare) cases where sysupgrade is
not possible at all, because it would break the device.
In this case, a warning will be printed, and -n won't help.

If image check fails because of one of the versions parts not
matching, the content of DEVICE_COMPAT_MESSAGE is printed in
addition to the generic message (if set).

For both cases, upgrade can still be forced with -F as usual.

Signed-off-by: Adrian Schmutzler <freif...@adrianschmutzler.de>
---
 .../base-files/files/lib/upgrade/fwtool.sh    | 20 +++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/package/base-files/files/lib/upgrade/fwtool.sh 
b/package/base-files/files/lib/upgrade/fwtool.sh
index a0b3fb0a04..dc064fee80 100644
--- a/package/base-files/files/lib/upgrade/fwtool.sh
+++ b/package/base-files/files/lib/upgrade/fwtool.sh
@@ -44,6 +44,26 @@ fwtool_check_image() {
        }
 
        device="$(cat /tmp/sysinfo/board_name)"
+       devicecompat="$(uci -q get system.@system[0].compat_version)"
+       [ -n "$devicecompat" ] || devicecompat="1.0"
+
+       json_get_var imagecompat compat_version
+       json_get_var compatmessage compat_message
+       [ -n "$imagecompat" ] || imagecompat="1.0"
+
+       # major compat version -> no sysupgrade
+       if [ "${devicecompat%.*}" != "${imagecompat%.*}" ]; then
+               echo "This image is incompatible for sysupgrading based on the 
image version."
+               [ -n "$compatmessage" ] && echo "$compatmessage"
+               return 1
+       fi
+
+       # minor compat version -> sysupgrade with -n required
+       if [ "${devicecompat#.*}" != "${imagecompat#.*}" ] && [ "$SAVE_CONFIG" 
= "1" ]; then
+               echo "This config is incompatible to the new image. Please 
upgrade with -n"
+               [ -n "$compatmessage" ] && echo "$compatmessage"
+               return 1
+       fi
 
        json_select supported_devices || return 1
 
-- 
2.20.1


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